Floating production platform

ABSTRACT

APPARATUS INCLUDING A SUBSTANTIALY VERTICAL ANNULAR BOUYANCY CHAMBER FORMED BY CONCENTRIC CYLINDRICAL SHELLS, THE INTERIOR SURFACE OF WHICH FORMS A CYLINDRICAL STORAGE CHAMBER THAT IS OPEN AT ITS LOWER END. A PLURALITY OF CIRCUMFERENTIALLY SPACED VERTICAL SUPPORT MEMBERS ARE MOUNTED ON THE UPPER END OF THE BOUYANCY CHAMBER AND SUPPORT A DECK. THE DECK IS ADAPTED TO HAVE PRODUCTION EQUIPMENT AND TANKER MOORING APPARATUS MOUNTED ON IT. THE BOUYANCY CHAMBER AND ASSOCIATED STRUCTURE MAY BE MOORED TO A SUBMERGED BOTTOM.

March 23, 1971 p, KNAPP EI'AL 3,572,278

FLOATING PRODUCTION PLATFORM Filed Nov. 27. 1968' 2 Sheets-Sheet I R/CHARD P KNAPP KENNETH T KOONCE IN VliN 0R3 MKW A T TORNE Y March 23, 1971 P, KNA P ETAL 3,572,278

FLOATING PRODUCTION PLATFORM Filed NOV. 27, 1968 2 Sheets-Sheet 2 FIG. 3

RICHARD P. KNAPP KENNETH T KOONCE INVBN'IORS ATTORNEY United States Patent 01 3,572,278 FLOATING PRODUCTION PLATFORM Richard P. Knapp and Kenneth T. Koonce, Houston, Tex., assignors to Esso Production Research Company Filed Nov. 27, 1968, Ser. No. 779,407 Int. Cl. 1363b 35/44 US. Cl. 114.5 13 Claims ABSTRACT OF THE DISCLOSURE Apparatus including a substantially vertical annular buoyancy chamber formed by concentric cylindrical shells, the interior surface of which forms a cylindrical storage chamber that is open at its lower end. A plurality of circumferentially spaced vertical support members are mounted on the upper end of the buoyancy chamber and support a deck. The deck is adapted to have production equipment and tanker mooring apparatus mounted on it. The buoyancy chamber and associated structure may be moored to a submerged bottom.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to offshore production platforms and is particularly concerned with deep water floating platforms that support production equipment, oil storage facilities, and tanker mooring apparatus.

(2) Description of the prior art Frequently, it is impractical to construct a pipeline to transport olfshore oil production to refineries and so tanker transportation is required. For example, there may be no market near shore, the marine environment may make pipelines unfeasible or too costly, or the size of the oil reserve may not justify the cost of a pipeline. Use of tankers to transport the crude oil normally requires construction of production storage facilities at the offshore location having sufficient capacity to retain continuous oil production and having apparatus necessary to rapidly load tankers. In shallow water, pile supported production platforms have been constructed which have sufficient crude oil storage to permit continuous oil production and which have apparatus for off-loading the oil onto small tank vessels. The use of such bottom-founded platforms for storage of large volumes of oil in deep water offshore locations poses serious problems.

Because deep water bottom-founded platforms are taller and more massive than their shallow water counterparts, they ordinarily have a substantially greater surface area exposed to the forces of the sea. So, forces caused by wind, waves, and currents are substantially increased and result in very large moments acting on the base of the legs as a result of the additional surface area and height of the platform. In addition conventional platforms are only able to support some 2-4 thousand tons and are thus wholly inadequate for supporting 200,000 barrels of oil, which would weigh approximately 25,000 tons. Resultant design requirements make deep water bottom-founded platforms extremely expensive. A need thus exists for a deep-water production and oil storage facility that will withstand forces caused by winds, waves, and currents and is more economical to construct than a bottom-founded platform.

SUMMARY OF THE INVENTION The floating production platform of the invention can be used to store petroleum offshore in deep water, can be made to withstand the forces created by wind, waves, and ocean currents, and yet can be constructed for a reasonable cost. The platform of the invention includes 3,572,278 Patented Mar. 23, I971 two concentric vertical cylindrical shells connected together to form an annular buoyancy chamber. The surface of the inner shell serves as the wall of a substantially cylindrical central oil storage chamber that is open at its lower end. A plurality of support members extend upwardly from the top of the buoyancy chamber and a deck is mounted atop the support members. Means are provided for mooring the platform to a submerged bottom.

The buoyancy chamber is preferably cylindrical, being formed by two concentric cylindrical shells, so that a single hollow interior storage chamber is formed within it. A plurality of vertical cylindrical buoyancy chambers may also be used and will reduce the area of the structure that is exposed to wind, waves, and ocean currents. The deck mounted on top of the vertical support members forms a convenient support for production equipment, off-loading apparatus, and equipment necessary for mooring a tanker during off-loading operations. In some applications it will be desirable to install a floating roof on the cylindrical storage chamber. In other applications a fixed roof over the storage chamber will be desirable. For example, a fixed roof is particularly desirable if the production platform is moored by tension members at a draft such that the storage chamber is completely submerged. It will thus be seen that the floating production platform of the invention has significant advantages over bottom-founded production platforms.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a conventionally moored floating production platform having a single annular buoyancy chamber constructed in accordance with the invention.

FIG. 2 is a perspective view of a conventionally moored floating production platform having a plurality of annular buoyancy chambers constructed in accordance with the invention.

FIG. 3 is a perspective view of a floating production platform constructed in accordance with the invention that has a single annular buoyancy chamber and that is moored in a submerged position by tension members.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The floating production platform shown in FIG. 1 has a substantially vertical annular cylindrical buoyancy chamber 10, the interior of which forms storage chamber 12 that is open at its lower end. The vertical buoyancy chamber is formed by two concentric cylindrical shells which may be connected together by annular plates or other suitable structural members. The cross-section of the buoyancy chamber may form a circle, an ellipse or some other closed curve. The buoyancy chambers should be sized such that the structure will float at a draft that will keep the deck above the level of wave action and the oil level below the top of the buoyancy chamber. The exterior surface of the inner cylindrical shell of the annular buoyancy chamber forms the wall of a central cylindrical storage chamber. Although the primary purpose of the storage chamber is crude oil storage, it will be apparent that it could also be used for other fluids. This storage chamber is open at its lower end and may be open or closed at the top.

A roof is preferably used to cover the top of the storage chamber and may be constructed of steel, polypropylene, or some other suitable material; however, if a fixed roof is used, vents should be provided to allow air to move in and out of the cylinder as the level of the oil varies. The buoyancy of the chamber will remain virtually constant during the passage of waves, even when a roof is used,

since the oil level is free to rise and fall and the small closed annular ring provides very little area upon which pressure variations caused by the passage of a wave can act. To provide safety from the hazard of hydrocarbon vapors above the oil, a floating top 13 constructed of plastic or aluminum or any other light material may be provided. Provision may also be made to maintain a layer of gas highly concentrated with carbon dioxide in the vicinity of the top. Because CO is heavier than air, it will tend to settle at the oil surface. As the CO cushion is diluted by diffusional mixing with the air, CO inlets in the top can be used to inject sufflcient CO to reconcentrate the vapor cushion to the desired concentration level. As an added precaution, a ring of foam inlets may be installed around the inside periphery of the tank near the top to completely fill the storage cylinder air space with a fire extinguishing foam if a fire should occur.

Support members 14 are circumferentially spaced about the upper end of the buoyancy chamber. These support members are preferably cylindrical; however, other configurations of structural members can also readily be used. A sufficient number of vertical members to support the production platform must be included in the structure. In determining the number of support members required, consideration should be given to the additional stresses that will be imposed on these members as a result of mooring shuttle tankers or similar vessels to the platform. Also, if the diameter of the vessel is very large, it may be desirable to install some trusses to provide additional support for the deck load.

The platform deck 18 is mounted on the upper end of the vertical support members and is adapted to support production equipment, crew quarters, and equipment for mooring a tanker to the floating platform and off-loading crude oil to it. FIG. 1 illustrates positioning of liquid-gas separation equipment 32, storage tanks 30, crew quarters 34, and a crane 36. Shuttle tankers may be moored at the platform by a rotatable, hinged, shock-absorbing, mooring arm 38. A circular track 40, preferably around the perimeter of the top of the buoyancy chamber, will permit the tanker 42 to be moored at the platform headed into the prevailing winds, waves, or currents and to rotate about the platform as seas change. Oil transfer pumps, not shown, are preferably mounted on the production platform, but could also be mounted on the tanker. Connection from the storage chamber to the tanker is by means of a flexible loading hose 44 or other flexible conduit. Preferably oil is conducted to the deck level from the storage chamber through the hollow interior of one or more of the support members.

Means for anchoring the structure to a submerged bottom are shown as a series of spaced anchors 22 which are attached to the submerged bottom, Chains, wire rope, cables 20, or other tension members may be attached to the platform deck and may depend downwardly through the annular buoyancy chamber or the cylindrical storage chamber. At a suitable point below the Water surface the chains emerge through ports 48 in the wall of the buoyancy chamber. The tension members extend downwardly from these ports to anchors, piles or other anchor members on the subsea bottom.

A suitable location may be selected for the floating production platform after a number of subsea oil wells 24 are completed. The platform, which is constructed on shore, is towed by a surface vessel to the location. It may be desirable to introduce water into the buoyancy chamber to give the platform a deeper draft and thus additional stability during towing. Alternatively, it may be desirable to tow the structure in a horizontal position and rotate it to an upright position at the location by the use of sea water ballast. Once on location, anchors 22 may be implanted in the submerged bottom in a pattern around the location where the floating production platform is to be moored, the platform centered within the anchor pattern, and cables 20 lowered from the platform and connected to the anchors. Necessary subsurface connections may be performed by divers. A preferred procedure, however, is to connect anchors and chains on the deck of an auxiliary vessel and then lower each anchor to the submerged bottom with a pendant line. With the mooring established, water is discharged from the buoyancy chambers so that the production platform will assume its operating draft.

Flowlines 26 from underwater wells 24 are extended to a central location where they may be connected to flexible riser hoses 28. The flexible risers extend upwardly through the storage chamber to a manifold on platform 18. The flexible riser hoses may also be connected to rigid conduits at the bottom of the buoyancy chamber which are in turn connected to a manifold on the platform, Fluid produced from the underwater wells flows through the lines to the manifold. The well fluid is transferred from the manifold to an oil and gas separator 32 which discharges the oil into temporary storage tanks 30. From the storage tanks, the oil is transferred to storage chamber 12 preferably down a hollow support column 14. Periodically, a tanker 42 is moored at the production platform by means of mooring arm 38 and a loading hose is connected to the tanker. Oil may be pumped from cylindrical storage chamber 12 up through a hollow support member 14, through loading hose 44, and onto tanker 42 for transportation to market.

An alternate configuration of the floating production platform is shown in FIG. 2. This embodiment includes a plurality of vertical cylindrical shells connected together concentrically in pairs to form an annular buoyancy chamber between each pair of shells 100, and a substantially cylindrical oil storage chamber in the center of each pair 102. The storage chambers are open at their lower ends. A plurality of support members 104 extend upwardly from the tops of the buoyancy chambers. The platform deck 106 is mounted on top of the support members. Construction, installation and operation of the floating platform of FIG. 2 are very similar to corresponding functions discussed in relation to the production platform of FIG. 1, The primary advantage of a platform such as is shown in FIG. 2 is the relative transparency of the structure to wave action afforded by the plurality of buoyancy chambers.

Another alternate configuration of the floating production platform of the invention is depicted in FIG. 3. This structure includes a submerged, substantially vertical annular buoyancy chamber 120, the interior of which forms a storage chamber 122 that is open at its lower end. The top of the storage chamber 124 is closed. A plurality of circumferentially spaced vertical support members 125 are shown mounted on the upper end of the chamber to support platform deck 128. Since the intensity of wave action decays rapidly with depth, normally only the vertical cylindrical columns are subjected to significant wave forces. Thus, this configuration of the invention is least affected by wave action. A major difference between the floating production platform of FIG. 3 and those in FIG. 1 and FIG. 2 resides in the means for mooring the platform to a submerged bottom, The buoyancy chamber of FIG. 3 is submerged at a desired draft, has a positive buoyancy and is moored to anchor members, not shown, by taut vertical tension members 132 which may be cables, rods, chains, bridge strands or the like.

What is claimed is:

1. A floating production platform comprising:

(a) two concentric vertical cylindrical shells connected together to form an annular buoyancy chamber and a substantially cylindrical central storage chamber open near its lower end;

(b) means for permitting pressure communication between the upper end of said storage chamber and the atmosphere, said means permitting the level of stored fluid to rise and fall within said chamber without substantially affecting the buoyancy of said production platform;

(c) a plurality of support members extending upwardly from the upper end of the buoyancy chamber;

(d) a deck mounted atop the support members; and

(e) means for mooring the platform to a submerged bottom.

2. A floating production platform as defined by claim 1 wherein said means for permitting pressure communication between the upper end of said storage chamber and the atmosphere includes a means for covering said storage chamber.

3. A floating production platform as defined by claim 2 including:

(a) at least one conduit extending downwardly from the deck for connection to a subsea well;

(b) separating means on the deck for separating well fluids produced through the conduit; and

(c) means for transferring fluid from the separating means to the storage chamber.

-4. A floating production platform as defined by claim 3 including:

(a) means on the platform for mooring a vessel to the structure; and

(b) means for off-loading fluid from the storage chamber to the vessel, including a flexible conduit.

5. A floating production platform as defined by claim 4 wherein the means for mooring the vessel to the structure includes a hinged, shock-absorbing, mooring arm that is rotatable around the platform.

6. A floating production platform as defined by claim 4 wherein the means for mooring a vessel to the structure includes a circular track that extends around the perimeter of the platform and a mooring arm movable along the track.

7. A floating production platform as defined by claim 2 wherein said covering means is a floating roof disposed within the cylindrical storage chamber and slidable relative to the wall thereof.

8. A floating production platform comprising:

(a) a plurality of vertical, cylindrical shells concen-' trically connected in pairs to form an annular buoyancy chamber between each pair of shells and a substantially cylindrical oil storage chamber open at its lower end in the center of each pair;

(b) means for permitting pressure communication between the upper end of each said storage chamber and the atmosphere, said means permitting the level of stored fluid to rise and fall within each said chamber without substantially affecting the buoyancy of said production platform;

(c) a plurality of support members extending upwardly from the upper ends of the buoyancy chambers;

(d) a substantially horizontal deck mounted atop the support member; and

(e) means for mooring the platform to a subsea bottom.

9. A floating production platform as defined by claim 8 wherein said means for permitting pressure communication between the upper end of each said storage chamber and the atmosphere includes a means for covering each said storage chamber.

10. A floating production platform as defined by claim 9 including:

(a) at least one conduit extending downwardly from the deck to a subsea well;

(b) separating means on the deck for recovering oil from the well fluids produced through the conduit; and

(c) means for transferring oil from the separating means to the oil storage chambers.

11. A floating production platform as defined by claim 10 including:

(a) means on the platform for mooring a vessel to the structure, said means including a track around the perimeter of the structure a mooring arm positioned on said track; and

(b) means for off-loading oil from the storage chamber to the vessel, said means including a pump and a flexible conduit.

12. A floating production platform as defined by claim 11 wherein said mooring arm is a hinged, shock-absorbmg, rotatable mooring arm provided with means for moving around the track.

13. A floating production platform comprising:

(a) two parallel concentric cylindrical shells connected together at the top and bottom to form an annular buoyancy chamber and a cylindrical central oil storage chamber that is open at its lower end;

(b) a plurality of vertical support members mounted on and circumferentially spaced about the upper end of the buoyancy chamber;

(0) a substantially circular, horizontal, floatable roof disposed within the cylindrical storage chamber and slidable relative to the wall thereof;

((1) a deck mounted atop the support members;

(e) at least one conduit extending downwardly from the deck to a subsea well;

(f) separating means on the deck for recovering oil from well fluids produced through the conduit; (g) means for transferring oil from the separating means to the oil storage chamber;

(h) means for mooring a vessel to the platform including a circular track that extends around the perimeter of the platform and a hinged, shock-absorbing, mooring arm that is mounted on the track and rotatable about the platform;

(i) means including a pump and flexible conduit for off-loading oil from the storage chamber to the vessel; and

(j) means including a plurality of tension members and a plurality of anchor members for mooring the platform to a submerged bottom.

References Cited UNITED STATES PATENTS TRYGVE M. BLIX, Primary Examiner US. Cl. XJR. 

